Patent GB-877506-A (GB)
Title: Electron beam tube
IPC: H01J31/04
Filed: 1958-07-14
Abstract: 877,506. Electron beam apparatus. EITELMcCULLOUGH Inc. July 14, 1958 [Oct. 23, 1957], No. 22478/58. Class 39(1). An electron beam switching tube has a gun anode 13 and collector 61 each of which has an annular terminal sealed to ceramic envelope portion 22 the transverse plane lying midway of the length of the envelope portion 22 coinciding with the 50% equipotential surface between the anode and collector at least at the outer surface of the insulating cylinder. The voltage breakdown strength between the gun anode and collector terminal assemblies is thus increased. The potential gradient along the ceramic envelope cylinder 22 is uniform and the seals 24, 26 are of special curved construction both factors further increasing the voltage breakdown strength, and the curved seals also saving space in the axial direction. The whole tube is immersed in an oil bath to further increase the voltage breakdown strength. The 50% equipotential surface is planar (if electrodes 53, 72 are mirror images of one another) or irregular if they are not. The special seals comprise curved metal rings 32, 33; 38, 46; 39, 44; 51, 53 (the latter being for the focusing electrode) which are brazed to the terminal rings 28, 43, 17, to the metallised ends of the ceramic cylinders 22, 23 and to the metallised ends of the ceramic backing rings 34, 41, 42, 52. The brazing is effected by high temperature metal alloys. The metal sealing rings 32, 33 are bonded together at their outer edges. Apertures 37, 49, 56 in tie discs 28, 43, 17 relieve pressure on the seals during brazing and the tube is finally evacuated via exhaust tube 66 having a protective cap 67. The collector has a nearly closed hollow interior to trap secondary electrons. The other tube seals e.g. 28 to 29, 31 are also made by brazing. The opening 62 in part 63 of collector has a tapered and polished surface and anode annulus 71 has a rounded corona end 73. The gun anode 13 is used for accelerating the beam and as a current control or signal modulating. The ceramic parts are pre-metallised by the Mo-Mn powder sintering process electrode it also serves by choice of the length of annulus 71 as an electrostatic shield between cathode and collector and protects the cathode from positive ion bombardment. The gun has also focusing electrode 16. Envelope parts 22, 23 are of ceramic material preferably alumina. The oxide coated indirectly heated cathode has a concave spherical cap emissive surface. Focusing electrode disc 17 is sealed to a ceramic ring (not shown) by ring 18, the cathode has a disc terminal 19 and the heater a ring terminal 21. The gun focuses the electron beam down to a point of minimum diameter at about the gun anode annulus 71 so that the beam electrons do not appreciably strike it unless its potential is greater than collector potential. The gun anode 13 controls the current flow in the tube and for a particular value of gun anode voltage over a large range of collector voltage (e.g. 1-60 K.V.) the collector current remains constant. If the colector voltage is less than 1 K.V., electrons are repelled to the gun anode 13 so reducing the voltage of 13, so reducing the collector current and increasing the collector voltage so fewer electrons are repelled so the voltage of gun anode 13 goes up again; thus an equilibrium point is reached where the tube automatically adjusts the current flow. The tube may be used to charge a capacitor in a definite short time interval and to keep it charged for a desired time interval. For operation as a signal amplifier the input signal is applied to gun anode 13, the output being taken from the collector.